Where to Learn Cycles Material Nodes from basics
Each node is very basic and very descriptive. For example, A Mix node mixes 2 input based on a factor value, a SplitXYZ node splits the input into XYZ. A simple online search should help if you still need clarification.

I think what you need to learn is the high level concept of shading and texturing. Look up "PBR material" basics, That should get you started learning about how the shaders behave, how inputs such as albedo, normal, roughness affects the surface, and how to create steel, plastic, etc.

I also suggest you focus on learning about the principled shader(as oppose to individual shaders such as glossy or diffuse), as it's the most artist friendly, complete, and industry-accepted pbr shading model.